This invention relates to a process for separating natural gas liquids from liquefied natural gas (LNG) and using the low LNG temperature to produce power. The process also vaporizes the LNG to produce natural gas meeting pipeline specifications.
It is well known that LNG in many instances when vaporized does not meet pipeline or other commercial specifications. The resulting natural gas may have an unacceptably high heating value, which may require dilution of the natural gas with materials such as nitrogen. The separation of nitrogen from the air to produce this diluent adds an expense to the natural gas. Alternatively, natural gas liquids may be removed from the LNG to produce natural gas having a heating value within the specifications for a pipeline. The natural gas liquids (NGLs) typically comprise hydrocarbons containing two or more carbon atoms. Such materials are ethane, propane, butanes and, in some instances, possibly small quantities of pentanes or higher hydrocarbons. These materials are generally referred to herein as C2+ materials. These materials not only add heating value to the natural gas which may increase its heating value beyond specification limits, but they also have greater value in their own right as separately marketable materials. It is desirable in many instances to separate these materials from natural gas prior to vaporizing it for delivery to a pipeline or for other commercial use.
In many instances in the past, LNG has been vaporized by simply burning a portion of the vaporized LNG to produce the heat to vaporize the remainder of the LNG and produce natural gas. Other heat exchange systems have also been used.
These systems require the consumption of substantial energy which may be produced as indicated by consumption of a portion of the product for vaporization, for distillation, for the production of nitrogen for use as a diluent and the like.
Accordingly a considerable effort has been directed toward the development of processes, which are more efficient for accomplishing this objective.
According to the present invention, it has been found that LNG is readily vaporized and NGLs removed therefrom by a process comprising: vaporizing at least a major portion of a stream of the liquefied natural gas to produce an at least partially vaporized natural gas stream; fractionating the at least partially vaporized natural gas stream to produce a gas stream and a natural gas liquids stream; compressing the gas stream to increase the pressure of the gas stream by about 50 to about 150 psi to produce a compressed gas stream and cooling the compressed gas stream by heat exchange with the stream of liquefied natural gas to produce a liquid compressed gas stream; pumping the liquid compressed gas stream to produce a high-pressure liquid stream at a pressure from about 800 to about 1200 psig; vaporizing the high-pressure liquid stream to produce a conditioned natural gas suitable for delivery to a pipeline or for commercial use; and recovering the natural gas liquids.
It is further been found that the LNG may be vaporized, NGLs may be recovered and substantial power may be recovered from the vaporization and separation process by vaporizing at least a major portion of a stream of the liquefied natural gas to produce an at least partially vaporized natural gas stream; fractionating the at least partially vaporized natural gas stream to produce a gas stream and a natural gas liquids stream; compressing the gas stream to increase the pressure of the gas stream by about 50 to about 150 psi to produce a compressed gas stream and cooling the (compressed gas stream by heat exchange with the stream of liquefied natural gas to produce a liquid compressed gas stream; Pumping the liquid compressed gas stream to produce a high-pressure liquid stream at a pressure from about 800 to about 1200 psig; vaporizing the high-pressure liquid stream to produce a conditioned natural gas suitable for delivery to a pipeline or for commercial use; recovering the natural gas liquids; passing at least one of a first portion and a second portion of a gas heat exchange fluid in heat exchange contact with at least one of the stream of liquefied natural gas and the high-pressure liquid steam to produce a liquid heat exchange fluid; pumping the liquid heat exchange fluid to produce a high-pressure liquid heat exchange fluid; heating the high-pressure liquid heat exchange fluid to vaporize the high-pressure liquid heat exchange fluid to produce a high-pressure gas heat exchange fluid; driving an expander and electric power generator with the high-pressure gas heat exchange fluid to produce electric power and the gas heat exchange fluid; and, recycling the gas heat exchange fluid to heat exchange with the at least one of the streams of liquefied natural gas and the high-pressure liquid stream.
It is further been found that the LNG may be vaporized with the recovery of NGLs and conditioned for delivery to a pipeline or for commercial use by a process comprising: vaporizing at least a major portion of a stream of the liquefied natural gas to produce an at least partially vaporized natural gas stream; separating the at least partially vaporized natural gas stream into a gas stream and a liquid stream; compressing the gas stream to increase the pressure of the gas stream by about 50 to about 150 psi to produce a compressed gas stream; fractionating the liquid stream at a pressure greater than the pressure of the compressed gas stream to produce an overhead gas stream and a natural gas liquids stream; recovering at least a portion of the natural gas liquids stream; combining the overhead gas stream with the compressed gas stream to produce a combined gas stream; cooling the combined gas stream by heat exchange with the stream of liquefied natural gas to produce a liquid stream; pumping the liquid stream to produce a high-pressure liquid stream at a pressure from about 800 to about 1200 psig; and, vaporizing the high-pressure liquid stream to produce a conditioned natural gas stream suitable for delivery to a pipeline or for commercial use.
It has further been found that the natural gas may be vaporized, NGLs recovered and the natural gas resulting from the vaporization of the LNG may be conditioned for delivery to a pipeline or for commercial use with the concurrent generation of electrical power by vaporizing at least a major portion of a stream of the liquefied natural gas to produce an at least partially vaporized natural gas stream; separating the at least partially vaporized natural gas stream into a gas stream and a liquid stream; compressing the gas stream to increase the pressure of the gas stream by about 50 to about 150 psi to produce a compressed gas stream; fractionating the liquid stream at a pressure greater than the pressure of the compressed gas stream to produce an overhead gas stream and a natural gas liquids stream; recovering the natural gas liquids stream; combining the overhead gas stream with the compressed gas stream to produce a combined gas stream; cooling the combined gas stream by heat exchange with the stream of liquefied natural gas to produce a liquid stream; pumping the liquid stream to produce a high-pressure liquid stream at a pressure from about 800 to about 1200 psig; vaporizing the high pressure liquid stream to produce a conditioned natural gas stream; passing at least one of a first portion and a second portion of a gas heat exchange fluid in heat exchange contact with at least one of the liquefied natural gas streams and the high-pressure liquid stream to cool the gas heat exchange fluid to produce a liquid heat exchange fluid; heating the high-pressure liquid heat exchange fluid to a temperature to vaporize the high-pressure liquid heat exchange fluid to produce a high pressure gas heat exchange fluid; driving an expander and electric power generator with the high-pressure gas heat exchange fluid to produce electric power and the gas heat exchange fluid; and, recycling the gas heat exchange fluid to heat exchange with the at least one of the liquefied natural gas stream and the high-pressure liquid stream.
Further, the present invention comprises: a liquefied natural gas inlet line in fluid communication with a liquefied natural gas source and a first heat exchanger; a distillation column in fluid communication with the first heat exchanger and having a gaseous vapor outlet and a natural gas liquids outlet; a compressor in fluid communication with the gaseous vapor outlet and a compressed gas outlet; a line in fluid communication with the compressed gas outlet and the first heat exchanger; and a pump in fluid communication with the first heat exchanger and a second heat exchanger.
The invention further comprises: a liquefied natural gas inlet line in fluid communication with a liquefied natural gas source and a first heat exchanger having a heated liquefied natural gas outlet; a separator vessel in fluid communication with the first heat exchanger and having a separator gas outlet and a separator liquids outlet; a pump in fluid communication with the separator liquids outlet and having a high-pressure liquid outlet; a distillation column in fluid communication with the high-pressure liquid outlet from the pump and having an overhead gas outlet and a natural gas liquids outlet; a compressor in fluid communication with the separator gas outlet and a compressed gas outlet; a line in fluid communication with the compressed gas outlet and the overhead gas outlet to combine the compressed gas and the overhead gas to produce a combined gas stream and to pass the combined gas stream to the first heat exchanger to produce a higher-pressure combined gas liquid stream; and, a pump in fluid communication with the first heat exchanger and a second heat exchanger, the second heat exchanger being adapted to at least partially vaporize the higher-pressure combined gas liquid stream.
The invention further optionally comprises the use of a heat exchange closed loop system in heat exchange with at least one of a charged LNG stream to the process and a conditioned LNG product of the process.